Popular email programs like Outlook Express, Microsoft Outlook, or Windows Live Mail, and webmail providers like Hotmail, Yahoo Mail or Gmail, all allow you to set the "importance" or "priority" of email messages you send (how the feature is called depends on the email program or provider, but they are one and the same). The side screenshot shows you three emails in Hotmail, each with a different importance: Low Importance, No Importance set (default), and High Importance. In some cases, you may find up to 5 different "importance levels" at your disposal. But since three is the norm, only these three settings are guaranteed to be shown to all; the screenshot below shows the 5 "Priority" options available in the message Options menu for Mozilla Thunderbird:
Before delving into the email etiquette implications of using importance flags for your emails, let's answer the question by backtracking: it is assumed that, at least in a business environment, you will only take up other people's time by sending appropriately important messages. As this already implies, you should not send any email with an importance level or "Low". Instead, if the matter isn't urgent, consider mentioning it in person to the concerned parties during a break, or when you next cross each other in the hallways or parking lot.
When manually setting the importance or priority setting of an email message to High, here is a consideration, followed by a rule of thumb:
Summary: since you should not send low importance messages, we're left with high importance emails. This setting needs to be used sparingly to retain its "unusually important" perception - no more than one of these per recipient, per week.